The present invention relates to plasma arc cutting torches, and more particularly to a plasma arc cutting torch that can operate above or below water to provide high quality cuts in metal, with reduced noise, airborne particulate pollution, ultra-violet (UV) radiation, and glare.
Without muffling, typical noise levels produced in cutting 1/2 inch mild steel with a 400 Amp nitrogen plasma arc torch are on the order of 110 dbA. This is clearly a high noise level which needs to be controlled. In this country, certain OSHA regulations require that noise levels be kept below 95 dbA with a 50% duty cycle, and many European countries require noise levels below 85 dbA. Plasma arc cutting also produces airborne pollutants, UV radiation, and glare at levels that can be bothersome at best, and health hazards at worst.
The methods of muffling plasma arc cutting torches that are currently known include a water-table, a low-velocity thick-walled water sheath, and submerging the workpiece in water.
Underwater plasma arc cutting has become a popular method for reducing noise, airborne particulate pollution, and UV radiation and glare. While environmental advantages of underwater cutting are clear, there are numerous drawbacks. Underwater cutting typically requires a 10 to 20% increase in power level with a 10 to 20% decrease in cutting speed. Cut quality is also reduced with an increase in adhering dross. In addition, the presence of water makes the sensing of obstructions and initial height much more difficult when using a positioning system based upon plasma vortex pressure such as described in U.S. Pat. No. 4,203,022 to Couch, Jr., et al.
To date, only non-reactive plasma-forming gases such as nitrogen have been used in underwater plasma arc cutting. This has been due, in part, to the lower currents associated with the plasma arc columns of reactive plasma-forming gases such as oxygen and air. Reactive gases are more susceptible to the effects of water near the plasma than non-reactive gases.
Other problems with current methods of underwater plasma arc cutting are that water continuously flows into the cutting zone which substantially degrades the quality of the cut, and gaseous cutting products such as hydrogen accumulate under the workpiece. The accumulation of hydrogen under the workpiece presents a dangerous situation because of its tendency to detonate in a sporadic and uncontrolled manner.
Alternatively, noise from above-water plasma arc cutting torches can be muffled by placing the workpiece on a water-table such as disclosed in U.S. Pat. No. 3,787,247 to Couch, Jr. However, the water on the underside of the plate and in the cut itself generally reduces the quality of the cut, and the hydrogen which accumulates under the workpiece creates a dangerous situation.
Noise can also be reduced through use of a muffling device as described in U.S. Pat. No. 3,833,787 to Couch, Jr. That device operates by providing a low velocity, thick-walled, cylindrical water sheath around the plasma arc column. However, problems similar to those observed in underwater cutting are exhibited; namely, the water flow interferes with the plasma, adversely affecting the cut. This problem is particularly acute when utilizing a reactive plasma-forming gas such as oxygen or air. Even the combination of the water-table and the low velocity water sheath is not sufficient to reduce noise to less than 90 dbA.
It is therefore a principal object of this invention to provide high speed plasma arc cutting underwater with low power consumption.
Another principal object of this invention is to reduce the noise, airborne pollutants, UV radiation, and glare of plasma arc cutting above water.
It is still another object to improve the sensing of obstructions and initial height when performing plasma arc cutting underwater.
A further object is to improve the quality of cuts made by a plasma arc torch underwater by allowing use of reactive plasma-forming gases.
It is still a further object to improve the quality of plasma arc cutting underwater or on a water-table.
Yet a further object is to prevent water from interfering with the plasma arc column while cutting.
A further object is to prevent hydrogen from accumulating on the underside of the workpiece.
A still further object is to provide a device which can be constructed relatively simply and economically, and can be easily adapted to or attached upon existing plasma arc torches.
Other objects will in part be apparent and in part pointed out hereinafter.